The invention relates generally to electronic printed circuit boards, and more specifically to via filling with solder in printed circuit boards.
Electronic components are typically assembled into complex circuits by mounting them on printed circuit boards. These printed circuit boards are usually flat nonconductive boards with one or more layers of a conductive material such as copper fixed on or in the printed circuit board. The layers of copper are etched or otherwise formed to specific shapes and patterns in the manufacturing process, such that the remaining conductive copper traces are routed to connect electrical components to be attached to the printed circuit board.
It is not uncommon for printed circuit boards such as these to have copper layers on both a top and bottom side of the circuit board, but also several layers of copper traces or patterns sandwiched at various depths within the circuit board itself. These traces allow greater flexibility in circuit routing, and usually allow designing a more compact circuit board for a particular circuit than would otherwise be possible. The various layers are sometimes dedicated to particular purposes, such as a ground layer that only serves to distribute ground or signal return connections to various components.
Multiple layers that serve to connect components to other components often must be connected to components attached to a different layer (top or bottom) or the layers must be attached to each other at selected points, requiring use of what are commonly known as vias. These vias typically are essentially small conductive plated-through hole elements oriented perpendicular to the top and bottom surfaces of a circuit board that extend through at least two conductive layers of the circuit board, and that electrically connect circuit traces on at least two of the conductive layers to each other.
Each of these plated-through hole-type vias are first formed as a copper plated-through hole using technology such as is commonly used to produce plated-through holes for receiving electronic component leads. The plated-through hole via is then filled with a polymer and capped at each end with a metal cap, forming a closed cylinder filled with polymer. The completed cylindrical via extends through at least two layers of a multi-layer circuit board, and typically has connections to electrical traces or conductors on at least two different levels of the circuit board.
But, creating vias in this manner requires accurate and complete curing of the polymers used to fill the plated-through vias. Otherwise, outgassing of improperly cured polymer can cause the circuit board to fail. In some applications with large circuit boards and fine-pitch vias and traces, nearly half of all produced circuit boards have been scrapped due to outgassing problems with circuit board via polymers that were less than ideally cured.
One solution is to replace plated-through hole-type vias with solid metallic microvias formed using laser technology and solid metal injected into the laser-drilled via hole. This method works well, but requires additional equipment and increases the cost of via production of a typical printed circuit board from about $10 to $45-50 per board.
What is needed is a method of creating a via using existing technology and equipment that eliminates the outgassing problems associated with polymer-filled plated-through hole vias.